In a fuel injection type of internal combustion engine, the combustion characteristics are in part determined by the injection timing. In a typical prior art four stroke engine, fuel injection is initiated approximately between 35.degree. of the crankshaft before top dead center (BTDC) and 5.degree. BTDC, during the compression stroke. When the timing is advanced (closer to 35.degree.), the combustion of the injected fuel is relatively complete because of the long length of time permitted for the combustion process. Consequently, the timing may be advanced in order to reduce the rate of fuel consumption and to reduce emission of unburned hydrocarbons. However, advanced timing produces high cylinder pressures and temperatures which result in high NO.sub.2 emissions, except at low engine loads when relatively little fuel is being burned.
When the timing is retarded (closer to 5.degree. BTDC), the NO.sub.2 emissions are reduced because most combustion occurs after top dead center. However, the emissions due to unburned hydrocarbons increase, but they are not as harmful as the NO.sub.2 emissions.
The timing that is normally selected is a compromise and is a value that produces low quantities of both types of emissions. There are however advantages in being able to vary the timing in response to different engine operating conditions. At idling and light loads (below approximately 1/4 full load) it is advantageous to advance the timing, whereas during normal load conditions it is advantageous to retard the timing.
U.S. Pat. No. 3,951,117 dated Apr. 20, 1976, and pending U.S. patent application Ser. No. 667,264, now U.S. Pat. No. 4,134,549 disclose a fuel supply system including means for varying the timing of the initiation of injection of the fuel, and the timing may be varied through an infinite number of steps. The injectors disclosed in the above patent and application operate such that injection is terminated when a moving member moves past and opens a spill port. This method of terminating injection has the disadvantages that injection pressure is lost as soon as the spill port is opened, and that the time of termination of injection cannot easily be adjusted.